Various optical multilevel modulation formats have been proposed to meet the increasing demand for capacity in optical communication systems. The Quadrature Phase Shift Keying, QPSK, modulation format has been investigated in long-haul transmission experiments due to its advantage of high nonlinear tolerance and better optical signal to noise ratio, OSNR, performance comparing with other alternative formats. Recently, optical differential 8-level phase shift keying, 8PSK, has been proposed to improve channel capacity in wavelength division multiplexed optical communication systems. It offers higher spectral efficiency than QPSK and can be used in high spectral efficiency direct detection or coherent detection optical communications systems.
The implementation of an 8PSK modulator typically uses an In-phase/Quadrature, IQ, QPSK modulator cascaded with a phase modulator. This structure is based on the fact that the symbols in the 8PSK constellation diagram can be divided in two QPSK constellation diagrams, rotated by π/4 with respect to one another. Hence, to generate 8PSK symbols, the device I/Q-modulates a continuous wave optical carrier signal to generate a symbol in a QPSK constellation diagram and then the phase modulator applied a phase shift to the modulated optical carrier signal to rotate the symbol by 0 or π/4, depending on the phase offset of the symbol to be transmitted.
The resulting 8PSK constellation diagram strongly depends on the driving voltage and the frequency response of the phase modulator. The voltage amplitude of the drive signal for the phase modulator has to be precisely tuned to obtain the desired phase shift of π/4. No technique to achieve automatic tuning of the driving voltage to meet this requirement has been reported, therefore phase shifts cannot be efficiently achieved, causing unwanted phase distortions within the 8PSK constellation diagram. Any bandwidth limitation or non-flat amplitude spectral response of the driving amplifier or of the phase modulator will translate directly into a deviation of the resulting 8PSK constellation diagram from the ideal one, and thus will impact on the performance of an optical communications system incorporating this type of 8PSK modulator, as reported by Takahide Sakamoto, et al, “Electro-optic synthesis of multi-level coherent signals”, OECC 2009, Paper 545.